Interesting...
I see the "mature" people as those who act dignified. They're consistent. They plug away and don't throw wild curveballs at things.
You see obama with that 'jackass' comment about kanye, followed by a look of fear on his face. He's not as controlled as someone like hilary or cheney who would have thought it - and said nothing.
And consitency matters too. Obama used every job to get to the next. Edwards did 1 term and quit. Palin lasted 60% of her single term. Or, you look at Mccain, Cheney, Pelosi - people who have done this 20+ years and will be doing it 20+ more.
so yeah, obama is cool, hip, whatever, but he's not mature like some other candidates, and I dont think anyone can argue that. There's just a level of reagan/dole-like statesmanship that he doesn't have. Clinton didn't have it, but he does now, so you can grow it.